Writing instrument



FIG. 1 is a front right perspective view of an ornamental design for a writing instrument according to the present invention;

FIG. 2 is a front elevational view thereof;

FIG. 3 is a back elevational view thereof;

FIG. 4 is a right side elevational view thereof;

FIG. 5 is a left side elevational view thereof;

FIG. 6 is a bottom plan view thereof;

FIG. 7 is a top plan view thereof; and,

FIG. 8 is a front right perspective view in an exemplary use position.

All portions shown in broken lines are environmental and illustrative only and form no part of the claimed design. 

The ornamental design for a writing instrument, as shown and described. 